Ultra-royalist and pro-coup groups have called on the US embassy not to support some "biased Thais" who criticise Thailand, singling out Human Rights Watch (HRW) representative Sunai Phasuk.
"The US embassy has met several red-shirt activists but they did not visit us at the People's Democratic Reform Committee [PDRC] stages, so we are here to explain our stance to them," activist monk Phra Buddha Isara said Thursday.
The now-dissolved PDRC galvanised the toppling of the previous administration which was led by the Pheu Thai Party and backed by the red-shirt movement. The group's protests in part led to the May 22, 2014 coup.
The monk was prominent in the movement. His protest at the embassy marks an increase in activity after several months of relative quiet.
Phra Buddha Isara and Rienthong Nanna, leader of the People's Organisation for Royal Thai Monarchy Protection, met US embassy representatives while some of their supporters gathered outside the embassy amid rain and heavy traffic to press their case. The monk later emerged from the embassy to meet and greet a few hundred of his supporters who were awaiting the results of the meeting.
"We want them to expel Sunai Phasuk from the HRW as this person has always expressed unfair and biased comments against Thailand," Phra Buddha Isara said.
"Of course, the American diplomats said they could not meddle with an NGO, but the HRW is based in their country, so they can take action," the monk added.
He said he has invited embassy officials to discuss "[related] issues" with him on future occasions.
Phra Buddha Isara alleged Mr Sunai only took sides with one political group -- "moaning when the red-shirt Peace TV was shut down but not caring how many casualties the PDRC suffered, criticising the lese majeste law so he must be in the same gang as the Nitirat group [of academics], which is politically lopsided and critical of the beloved institution".
Mr Sunai declined to comment on the allegations made by Phra Buddha Isara, saying his organisation would prefer he not discuss the matter.
Phra Buddha Issara said his demonstration was not a political one like those of other groups at the Democracy Monument which are still calling for elections, so he believed the government understood the feelings of "loyal subjects".
"Our presence doesn't represent the government either. It's the voice of the Thai people that Washington should take heed of," said the monk, after the crowd sang royalist songs in the park in the rain.
In the discussions with the US embassy officials, the monk made it clear that no one should support people who want to amend the lese majeste law or defame the Thai monarchy, or else they would be considered as interfering in Thai domestic affairs.
The monk and Dr Rienthong did not meet the new US ambassador Glyn Davies.
Dr Rienthong, also Mongkutwattana General Hospital director, said he had submitted to the US embassy the names of some people who he claimed had committed lese majeste and still live in the US.
"We will give them more names and organisations and will ensure the US won't allow those people to sabotage the revered institution," said the doctor, whose previous activist group was named the Rubbish Cleaning Organisation.
The monk has encouraged his supporters to gather at Siriraj Hospital on Saturday to show solidarity and loyalty to His Majesty the King.
Asked about the "Bike For Dad" event in December, he said he supported any activities that forge loyalty to the monarchy.